Long lines of voters have been reported across the country, including in the critical swing states of Ohio in the midwest and Florida in the southeast, where some people queued for hours to cast a ballot.
The longest and costliest presidential race in US history has lurched to a climax with opinion polls showing no daylight between the feisty Republican president and the patrician Democratic senator from Massachusetts.
Five late surveys gave Mr Bush a statistically insignificant lead while Fox News showed Mr Kerry leading by two points and the American Research Group had a 48-48 percent tie.
The first result in came from the mainly Republican New Hampshire hamlet of Dixville Notch that, as per tradition, voted just after midnight.
Mr Bush’s 19-7 margin was down slightly from the 21-5 score he chalked up in 2000.
Three states are generally seen as pivotal in the race: Florida, Ohio and Pennsyvlania in the East.
All are considered tossups, as are half a dozen other “battlegrounds” that could swing the balance.
Senator Kerry needs to win two out of the big three states to have a chance at the presidency.
For Mr Bush, the loss of two would make it difficult but not impossible for him to reach the threshold of 270 electoral votes.
Analysts have not ruled out a candidate losing the popular vote and winning the election again as Mr Bush did four years ago, or a 269-269 tie in the electoral vote that could force Congress to decide the outcome.
US voters will also decide the composition of Congress, where Republicans are defending a slim Senate majority, but are heavily favoured to keep control of the House of Representatives.
There are widespread fears the election could have the same result as in 2000, when a disputed recount in the state of Florida had to be settled by the US Supreme Court five weeks later.
“This election is in the hands of the people. And I feel very comfortable about that,” said Mr Bush as he voted in the fire department and ambulance terminal in his hometown of Crawford, Texas, accompanied by his wife and twin daughters.
“The people know I know how to lead. The people know I have a vision for the future of this country,” he said, taking a final swipe at Mr Kerry by chiding politicians who “take positions but they don’t take a stand.”
In a break with tradition, both candidates attended Election Day events in battleground states.
“We’re going to link hands and hearts and we’re going to take America to a better place. Let’s get the job done,” Mr Kerry told cheering supporters at an early rally in Wisconsin before heading home to Boston to vote.
The race, the first in 30 years conducted with US troops fighting abroad, boiled down to a heated debate over last year’s invasion of Iraq and a question of who could keep America safer from terrorists.
It has pitted candidates with sharply contrasting styles, temperaments and political philosophies.
Mr Bush, 58, the born-again Christian son of ex-president George HW Bush, campaigned as a “war president” ready to take the United States into battle alone if necessary to safeguard the country’s security.
60-year-old Senator Kerry, a decorated Vietnam veteran, has taken a more nuanced stand, preaching the need to repair alliances damaged by the Iraq war and reconstruct an international consensus on the global war against terrorism.
With the US job market still fragile and Mr Bush’s economic policies under attack, security was his strong suit as he relentlessly derided Mr Kerry as weak and waffling on defence.
But late polls showed his once-substantial lead on the issue melting amid more bad news from Iraq and the dramatic re-emergence of Al-Qaeda chief Osama bin Laden on videotape.
Some experts said turnout could approach the record modern turnout of 63 percent in 1960, providing a measure of encouragement to Mr Kerry who led Mr Bush 61-36 percent among self-described new voters, according to an ABC News Poll.